1. Field of the Invention
The invention relates to an axial sliding bearing and to a method for operating it.
2. Discussion of Background
Hydrodynamic sliding bearings are often used for the mounting of turbochargers. Fixed segment bearings are normally employed as axial sliding bearings. These are composed of a bearing body connected fixedly to the bearing housing and having a profiled annular surface and of a plane sliding surface of the so-called bearing collar, said sliding surface rotating with the shaft. The profiled side has a plurality of radial lubricating grooves and corresponding wedge and catch lopping surfaces (see DE-A1 32 44 893). Said side may likewise be arranged on the bearing collar.
When the turbocharger is in operation, a loadbearing hydrodynamic lubricating film builds up between the plane surface and profiled surface on account of the shearing flow of the lubricating oil. The lubricating film thickness depends, inter alia, on the bearing load and, in the case of high pressure conditions, may, at about 20 .mu.m, be very small.
As a rule, the lubrication of such an axial sliding bearing is carried out by the lubricating system of the internal combustion engine connected to the turbocharger. In this case, its relatively large lubricating oil stream is filtered only coarsely, that is to say with a mesh width of about 35 to 50 .mu.m. However, because of its high rotational speeds and the narrow lubricating gaps, the turbocharger requires lubricating oil of finer consistency. Separate filtration for the turbocharger is usually impossible or too complicated and therefore relatively expensive (see DE-A1 44 11 617).
Under high pressure conditions, the minimum lubricating film thickness may, particularly on the axial sliding bearing, be smaller than the largest dirt particles which pass through the engine filter. This may give rise to premature wear of the bearing surfaces, especially of the wedge surfaces. However, if the necessary wedge surface of the bearing falls below its minimum value, the latter is no longer operationally reliable. In this case, an early failure of the bearing must be expected. The axial sliding bearing of turbochargers is therefore a component with a high risk of wear.
DD 49,087 discloses an axial sliding bearing for turbomachines, which is composed of a bearing body connected fixedly to a bearing housing, of a bearing collar rotating with the shaft and of a lubricating gap formed between the two. The lubricating gap is formed between a profiled annular surface rotating about the shaft and a plane fixed sliding surface. The annular surface has a plurality of radially arranged lubricating oil grooves and, in each case, a wedge surface connected to these in the circumferential direction, the lubricating oil grooves and the wedge surfaces being arranged alternately to one another on the annular surface. The lubricating oil grooves are in each case delimited relative to the outside by a sealing web. Each sealing web possesses a scavenging orifice passing through it and leading radially outward.
However, when a turbomachine mounted in this way is in operation, a substantial part of the lubricating oil may flow off outward over the edge of the profiled annular surface, that is to say, more precisely, over the wedge surfaces. The pressure buildup in the lubricating gap is thereby reduced, so that the latter becomes smaller. This leads to increased abrasion by dirt particles and consequently to a reduction in the life of the bearing.
DE-U1 78 19 938 discloses an axial sliding bearing for exhaust gas turbochargers, in which a floating disk arranged between the fixed bearing body and the rotating bearing collar is guided on the turbocharger shaft. The floating disk has a multisurface geometry with spiral oil grooves which are arranged on both sides and which extend as far as a region of contact with the bearing body or the bearing collar. With the aid of these grooves, a lubricating film is built up in each case between the bearing body and the floating disk, on the one hand, and between the floating disk and the bearing collar, on the other hand.
In such a solution with a floating disk, both the power loss of the turbocharger is reduced and, on account of the double lubricating gap, the skew compensation capacity of its thrust bearing is increased. In addition, the relative speed due to the disk rotation is reduced to about half the rotational speed of the rotor and consequently the wear behavior of the thrust bearing is markedly improved. However, in the case of such a floating disk, the minimum lubricating film thickness may likewise be smaller than the largest dirt particles of the lubricating oil, so that, even when said floating disk is used, the disadvantages described above cannot be eliminated.